Tekashi69just got super up close and personal during a visit from his girlfriend ... and she's letting everyone know her man's holding up just fine in jail.

Tekashi's GF, Jade, proudly showed off this candid snap Monday night of Tekashi grabbing her butt as she plants a kiss on his cheek during a jail visit earlier in the day. Jade captioned the photo, "He's good luv, enjoy. #FreeDanny"

It's the first time we've seen 6ix9ine behind bars. We broke the story ... Tekashi was busted by the feds in November on racketeering charges, and he's facing life in prison.

As we reported ... Tekashi has a plan to get out of jail, and it's easy to see why he wants some time on the outside with his chick.